1. Field of the Invention
This invention relates to an electrophotographic photoreceptor having excellent function and in particular to an electrophotographic photoreceptor having better photoresponse characteristics, better stability during cycle operation and better resistance to environment.
2. Disclosure of the Related Art
With development-b of non-impact printer technology in recent years, the research development on electrophotographic printers using laser beam sources has been practiced actively. In these apparatuses, reduction in size and speeding-up of the apparatuses have been required and in addition photosensitive materials having high photosensitivity and high speed photoresponse have been required. In electrophotographic photoreceptors using conventional charge-transfer materials for a charge-transfer layer, however, there could not have been still obtained sufficient characteristics. Namely, the photoreceptors had a high residual potential and dark decay and were bad in stability during cycle operation or repeated use. In addition, it is necessary for the photoreceptor to have stable characteristics in a desired range of temperature and relative humidity in a practical application and further stable characteristics have been desired for the photoreceptor.
With respect to the high-speed photoresponse characteristics as mentioned above, the improvement thereof has been tried by developing new materials for the charge-transfer layer having high mobility or increasing the density or ratio of the charge-transfer material in the charge-transfer layer, to thereby improve the photoresponse characteristics. However, it was difficult to develop the new desired materials. Furthermore, since the density of the charge-transfer material in the charge-transfer layer is uniform in a three-dimensional direction in a uniformly dispersed system of the charge-transfer material in a bonding resin, the mobility is proportional to 3rd root of an average intermolecular distance (Leading Concept for Developing Better Charge Transportable Organic Materials; R. Takahashi et al., Electrophotography, Vol. 25, No. 3, 10(1986)). For this reason, even if the density of the charge-transfer material in the bonding resin was increased, the mobility was slightly improved or rather there was a problem in the practical use that strength of the film was deteriorated.
In view of the above-mentioned problem, it has been proposed to conveniently combine two or more of the charge-transfer materials each having specific features. For instance, in order to adjust the mobility and the stability during cycle operation, it has been known to combine two charge-transfer materials, i.e. a butadiene series compound and a hydrazone compound as represented by CT-1 and CT-2, respectively, in Table 1 as mentioned below (refer to U.S. Pat. No. 4,839,252). However, this prior art sacrifices the good mobility of the butadiene series compound for the stability during cycle operation.
As described above, remarkable improvement in performance of the photoreceptor could not have been achieved by using the known compounds and any combination thereof on the basis of the known methods and further it was difficult to develop the new materials. Under the circumstances, it is very important to make the best use of the characteristics of the known charge-transfer materials.